U.S. patent number 5,304,809 [Application Number 07/945,229] was granted by the patent office on 1994-04-19 for luminescent decay time measurements by use of a ccd camera.
This patent grant is currently assigned to Luxtron Corporation. Invention is credited to Kenneth A. Wickersheim.
United States Patent |
5,304,809 |
Wickersheim |
April 19, 1994 |
Luminescent decay time measurements by use of a CCD camera
Abstract
A video camera of a type using an array of charge coupled
devices (CCDs) is utilized to measure a condition, such as
temperature, by imaging onto the camera a luminescent signal which
contains information of the signal's decay time. The luminescent
decay time is measured by comparing, such as by ratioing, the
integrated signal values obtained in successive frames of operation
of the CCD. One application includes use to measure a
two-dimensional temperature distribution across a surface. The
surface of interest is either coated with a layer of the
luminescent material or emissions from the surface are imaged onto
a separate luminescent screen. Another application is as a
multiplexer and detector system for a large array of optical fiber
sensors, a luminescent signal from each of the sensors being imaged
through its respective fiber onto a unique one or more of CCD
photosites.
Inventors: |
Wickersheim; Kenneth A. (Menlo
Park, CA) |
Assignee: |
Luxtron Corporation (Santa
Clara, CA)
|
Family
ID: |
25482822 |
Appl.
No.: |
07/945,229 |
Filed: |
September 15, 1992 |
Current U.S.
Class: |
250/458.1;
250/459.1; 374/161; 374/E11.024 |
Current CPC
Class: |
G01N
21/6408 (20130101); G01K 11/20 (20130101) |
Current International
Class: |
G01K
11/20 (20060101); G01K 11/00 (20060101); G01N
21/64 (20060101); G01K 011/00 (); G01N
021/64 () |
Field of
Search: |
;250/459.1,458.1
;374/161 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Paul Kolodner and J. Anthony Tyson, "Microscopic fluorescent
imaging of surface temperature profiles with 0.01.degree.C
resolution." Appl. Phys. Lett., vol. 40, No. 9 (May 1, 1982) pp.
782-784. .
Wood, Physical Optics. 3rd edition (Place and date of publication
unknown) pp. 665-666. .
Sholes, R. R. et al., "Fluorescent decay thermometer with
biological applications", Rev. Sci. Instrum. 51(7), pp. 882-884,
(Jul. 1980). .
Jensen, E. M. et al., "Optical Technique for Measurement of
Currents Induced by Microwave Frequency Radiation: I. Basic
Technology and Instrument Design", pp. T15.1-T15.6. .
Urbach, F. et al., "The Observation of Temperature Distributions
and of Thermal Radiation by Means of Non-Linear Phosphors", Journal
of the Optical Society of America, vol. 39, No. 12, pp. 1011-1019,
(Sep. 12, 1949). .
Weber, M. J. et al., "Optical spectra and relaxation of Cr.sup.3+
ions in YA10.sub.3 ", Journal of Applied Physics, vol. 45, No. 2,
pp. 810-816, (Feb. 1974). .
Masi, C. G. et al., "Finding Board Faults with Thermal Imaging",
Test & Measurement World, pp. 109-121, (Mar. 1989). .
Gartenberg, E. et al., "Twenty-Five Years of Aerodynamic Research
with Infrared Imaging", Journal of Aircraft, vol. 29, No. 2, pp.
161-171, (Mar.-Apr. 1992). .
Noel, B. W., et al., "Two-Dimensional Temperature Mapping Using
Thermographic Phosphors", Los Alamos National Laboratory, pp. 1-15,
presented at the 'High Temperature Sensors Symposium, 177th Meeting
of the Electrochemical Society Montreal, Quebec, Canada, May 6-11,
1990, but unpublished. .
Bugos, A. R. et al., "Remote Sensing with Thermophosphors",
Sensors, pp. 17,19 and 20 (Mar. 1990)..
|
Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Majestic, Parsons, Siebert &
Hsue
Claims
It is claimed:
1. A method of measuring a condition of an object or environment,
comprising the steps of:
positioning in communication with said object or environment a
quantity of luminescent material that is characterized by emitting,
in response to a pulse of excitation radiation, luminescent
radiation having a rate of decay that is related to a level of said
condition,
directing repetitive pulses of excitation radiation against the
luminescent material, thereby to cause a resulting luminescent
radiation to follow a repetitive cycle of building up during the
excitation pulses and thence decaying between pulses at a rate
related to the level of said condition,
positioning in the path of said luminescent radiation at least one
radiation detector characterized by generating an electrical charge
that is related to a level of said radiation and accumulates the
charge for gated periods of time,
gating said at least one detector in synchronism with the
excitation radiation pulses and in a manner to obtain values
corresponding to the charge accumulated during two different
intervals relative to the luminescent material emission cycle,
comparing the accumulated charge values during said two different
intervals, and
converting the charge value comparison to a magnitude of said
condition.
2. The method of claim 1 wherein the accumulated charge obtained
during two different intervals relative to the luminescent material
emission cycle as part of the gating step includes timing said
intervals to occur during the same luminescent cycle.
3. The method of claim 2 wherein timing of said intervals is
further caused to occur during the decaying of the luminescent
radiation between excitation pulses.
4. The method of claim 1 wherein the intervals of the gating step
do not overlap each other with respect to the luminescent radiation
cycle.
5. The method of claim 1 wherein the repetitive pulse directing
step includes directing the repetitive pulses at periodic
intervals.
6. The method of claim 1 wherein the charge comparing step includes
taking a ratio of the accumulated charge values.
7. The method of measuring a condition of a plurality of areas of a
luminescent material that is characterized by emitting, in response
to a pulse of excitation radiation, luminescent radiation having a
rate of decay that is related to a level of said condition,
comprising the steps of:
directing repetitive pulses of excitation radiation against the
luminescent material, thereby to cause the luminescent radiation to
follow a repetitive cycle of building up during the excitation
pulses and thence decaying between pulses at a rate related to said
condition,
positioning in a path of said luminescent radiation an array of
charge coupled device elements,
imaging said luminescent radiation onto said element array with
said luminescent material areas matched to individual elements of
said charge coupled device array,
gating said charge coupled device array elements in synchronism
with each other and with the excitation radiation pulses to operate
during two different intervals relative to the luminescent material
emission cycle,
comparing an output during said two different intervals for the
individual charge coupled device array elements, and
converting the compared outputs to said condition, whereby the
condition comparisons from each of the charge coupled device array
elements corresponds to an area of the luminescent material.
8. The method of claim 7 wherein the condition being measured is
temperature and the individual areas of luminescent material are
adjacent each other across a layer of luminescent material that is
physically attached to a surface of an object, whereby the
temperature is being measured across the surface of the object.
9. The method of claim 7 wherein the condition being measured is
temperature and the individual areas of luminescent material are
adjacent each other in a layer of said luminescent material
extending across a screen that also has an infrared absorbing layer
in thermal contact with the luminescent layer, said method
comprising the additional step of imaging an infrared image of an
object surface onto said infrared absorbing layer, whereby the
temperature of the object surface is being measured
thereacross.
10. The method of claim 7 wherein the condition being measured is
temperature and said plurality of areas of luminescent material are
distributed in a plurality of temperature sensors carried by
lengths of optical fibers, and wherein the imaging step includes
imaging luminescence from free ends of the optical fibers onto said
element array, whereby the temperatures of the plurality of sensors
are measured.
11. The method of claim 7 wherein the luminescent radiation imaging
step includes imaging said luminescent material areas onto said
element array through a bundle of optical fibers by matching
individual of said optical fibers with individual elements of said
array of charge coupled device elements.
12. The method of claim 7 wherein the gating step includes timing
said two different intervals to occur during the same luminescent
material emission cycle.
13. The method of claim 12 wherein timing of said two different
intervals are further caused to occur during the decaying of the
luminescent radiation between excitation pulses.
14. The method of claim 7 wherein the intervals of the gating step
do not overlap each other with respect to the luminescent radiation
cycle.
15. The method of claim 7 wherein the repetitive pulse directing
step includes directing the repetitive pulses at periodic
intervals.
16. The method of claim 7 wherein the output comparing step
includes taking a ratio of the output during said two different
intervals.
17. A method of obtaining a visual image of a temperature profile
across an area of a surface, comprising:
coating said surface with a layer of luminescent material that is
characterized by emitting, in response to a pulse of excitation
radiation, luminescent radiation having a temperature related rate
of decay,
directing repetitive pulses of excitation radiation against the
luminescent material layer, thereby to cause the luminescent
material radiation emission to follow a repetitive cycle of
building up during the excitation pulses and thence decaying
between pulses an amount related to temperature,
imaging said luminescent emission onto a two dimensional array of
charge coupled device elements characterized by generating
individual element signals representative of an accumulation of
luminescent emission radiation intensity falling thereon over a
period of time, thereby to detect said luminescent material
emission from adjacent points of said surface area,
gating said charge coupled device elements relative to timing of
the excitation radiation pulses and during two different intervals
of the luminescent radiation emission cycle, thereby to generate
two individual charge coupled device element signals for a
luminescent radiation cycle,
comparing said two signals individually from the charge coupled
device elements,
converting the individual charge coupled device element signal
comparison into temperature, and
generating from the individual element temperature conversion a
visual image corresponding to temperature across said surface
area.
18. An imaging device, comprising:
a video camera including a two-dimensional array of charge coupled
device elements that are individually characterized by generating,
after repetitively occurring frames, a signal that is related to an
integrated value of electromagnetic radiation striking the element
during the preceding frame,
means including a timing circuit for controlling the timing of said
camera frames,
a source of excitation radiation for a luminescent material,
means synchronized with said camera frame controlling means for
causing said radiation source to emit a sequence of pulses with
time durations therebetween, and
means receiving the individual element signal outputs for comparing
successively occurring pairs of such signals therefrom.
19. An infra-red camera, comprising:
a video camera including a two-dimensional array of charge coupled
device elements that are individually characterized by generating,
after repetitively occurring frames, a signal that is related to an
integrated value of electromagnetic radiation striking the element
during the preceding frame,
means including a timing circuit for controlling the timing of said
camera frames,
a screen having an infra-red absorbing layer on one side thereof
and luminescent material on another side thereof, said luminescent
material being characterized by emitting, in response to a pulse of
excitation radiation, electromagnetic radiation having a rate of
decay that is related to the temperature of the luminescent
material,
means for imaging an object onto said screen's infra-red absorbing
layer, thereby to heat the screen in accordance with an infra-red
emission image of the object,
a source of excitation radiation directed at said screen's
luminescent material layer, thereby causing an electromagnetic
radiation emission therefrom with decay times that vary across the
screen in accordance with the screen's temperature,
means for imaging said luminescent material layer emission onto the
charge coupled device element array,
means synchronized with said camera frame controlling means for
causing said radiation source to emit a sequence of pulses with
time durations therebetween, and
means receiving the individual element signal outputs for comparing
successively occurring pairs of such signals therefrom.
20. A multi-channel condition measuring system, comprising:
a plurality of optical fibers having individual sensors at one end
thereof, the individual sensors including a quantity of luminescent
material that is characterized by emitting, in response to a pulse
of excitation radiation, luminescent radiation having a rate of
decay that is related to said condition,
an array of charge coupled device elements that are individually
characterized by generating, after repetitively occurring frames, a
signal that is related to an integrated value of electromagnetic
radiation striking the element during the preceding frame,
a source of excitation radiation for said luminescent material,
optical means positioned at ends of said optical fibers opposite
said sensors for coupling said excitation radiation thereinto and
directing the luminescence from their individual sensors onto
individual ones of said charge coupled device elements,
means including a timing circuit for controlling the timing of said
camera frames,
means synchronized with said camera frame controlling means for
causing said radiation source to emit a sequence of pulses with
time durations therebetween, thereby to cause the luminescent
material radiation emission to follow a repetitive cycle of
building up during the excitation pulses and thence decaying
between pulses in an amount related to a level of said condition,
and
means receiving the individual element signal outputs for comparing
successively occurring pairs of such signals therefrom.
21. The system according to claim 20 wherein said array includes a
two-dimensional array of said elements.
22. The system according to claim 20 wherein said array consists of
a linear array of said elements.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to techniques for measuring a
decaying electromagnetic radiation signal, and, more specifically,
to the measurement of a decay time of a luminescence signal that is
generated in a material exposed to some parameter to be measured
thereby. Measurement of temperature is extensively discussed herein
but the invention is not limited to temperature applications.
One such application is the remote, real-time measurement of
surface temperature distributions. In wind tunnel experiments with
aircraft models, for example, a sequence of thermal images of the
surface of the model aircraft may be acquired as part of the test
data. Changes in the temperature distribution across the surface of
an operating electronic component or complete circuit board is
another application. In the medical field, medical thermography
involves a thermal imaging of a skin area of a patient to provide
information helpful to diagnosing the patient's condition. One way
of doing this is to scan an infrared image of the surface across a
point detector having an appropriate response to obtain electronic
signals which are then used to display an image in visible light.
The significant disadvantage of this technique is the resultant
complexity inherent in the optomechanical and infrared detection
technology used for the imaging system in order to achieve good
speed, sensitivity and spatial resolution. Also, although direct
detection of the infrared image can provide an acceptable
qualitative visual representation of the temperature profile across
the surface being viewed, the absolute accuracy of the measurement
of temperature may be limited for a variety of reasons.
Another approach to observing and measuring the temperature across
the surface is to first convert the temperature variations of the
surface into thermally-encoded visible or near visible emissions
before the radiation is detected and electronically processed. One
such technique is to position a layer of luminescent material in
thermal communication with the surface, such as by coating the
luminescent material directly onto the surface. More conventional
and less expensive imaging devices may then be used to detect and
process the visible or near visible luminescent emissions. One such
technique is to coat the surface with conventional thermographic
phosphors and then detect the intensity of the luminescent image,
much like any other optical image. This provides a good visual
representation of temperature variations across the surface but
suffers from a limited range of measurement and inaccuracies when
quantitative measurements of temperature are desired. Thus, others
have suggested variations of the luminescent material plus further
optical processing of the luminescent image, such as by a
pixel-by-pixel ratioing of the intensities of two separate and
thermally dissimilar wavelength bands of luminescent emission.
Given the right optics and luminescent material, this ratio is
proportional to temperature. It has also been suggested to measure,
on a pixel-by-pixel basis across the image, the decay time of the
luminescence of a coating after the coating is excited by a pulse
of excitation radiation. The decay time of the luminescent emission
of selected materials is proportional to the temperature of the
luminescent material over a given range of interest.
However, a totally satisfactory approach to such luminescent image
decay time analysis does not yet exist. Therefore, it is a primary
object of the present invention to provide this needed solution. An
important goal of the present invention is a low-cost, simple,
reliable and easy to use system that gives fast, accurate
temperature measurements across a two dimensional luminescent
surface over a wide range of temperatures.
Use of the luminescent decay time technique for measuring the
temperature of a single small spot of luminescent material is
becoming widespread. Optical fiber temperature measuring systems
are commercially available. A very small quantity of luminescent
material is formed as part of a sensor at the free end of an
optical fiber, the other end of the optical fiber being connected
to a measurement instrument. The instrument repetitively sends
pulses of excitation radiation down the fiber and receives back
from the sensor, in between the pulses, the decaying luminescent
signal which is typically approximately exponential in time. A
quantity proportional to the luminescent decay time, which is
indicative of the temperature of the sensor, is then obtained by
one of several signal processing techniques. One such technique is
to measure the time it takes for the decaying intensity signal to
fall from one value to another value that is the first value
divided by e. This time is by definition the decay time of the
luminescence. Another technique is to integrate the decaying
luminescent signal over two different periods and then compare the
integrated values, such as by ratioing them. Yet another technique
is to digitize the decaying luminescent signal and subsequently
analyze the digitized data to determine the decay time from the
best fit of an exponential curve to the data samples.
Currently, a typical fiberoptic temperature measuring system
includes only one or just a few optical fiber probes connected to a
common optical instrument and signal processing system. It is
another object of the present invention to provide such a
instrument and system that can be used to multiplex hundreds or
even thousands of separate optical fiber probes, and thus provide
separate temperature measurements from each of the probes at a
reduced cost per probe relative to what is now possible.
SUMMARY OF THE INVENTION
These and additional objects are accomplished by the present
invention, wherein, briefly and generally, the luminescent
radiation is imaged onto a two dimensional array of charge coupled
devices (CCDs) of the type that is used as the "retina" in
industrial and commercial solid state video cameras, and which is
readily available through commercial supply channels. The way that
a CCD camera array is operated has been recognized to be especially
adapted for use in making measurements of the decay time of
luminescent radiation that strikes it. Each photosite of the array
integrates the amount of electronic charge that is generated by
incident light during a single video frame. The luminescent
material is chosen to have a range of decay times over a
temperature range of interest that allows two intervals of a
decaying signal to be integrated in successive video frames, either
from a single decaying function or from two immediately sequential
decaying functions. These two sets of integrals are then compared,
by example, by ratioing or taking the difference over the sum, and
this comparison is then converted to temperature by an empirically
developed function or lookup table. An excitation source directs
pulses of radiation against the luminescent material with a timing
that is precisely coordinated with the frame rate of the CCD
camera. A quantity proportional to the decay time of luminescent
radiation striking each photosite is constructed in substantially
real time every few video frames, the number of frames depending
upon the particular signal processing technique that is chosen to
be implemented.
Such a CCD array operated in this manner is used for non-contact
surface temperature measurements by imaging onto the array
emissions from a thin luminescent layer whose thermal pattern
corresponds to that of the surface of interest, either by direct
contact of the luminescent layer with the surface or by imaging the
emissions from the surface onto a thin luminescent screen in a
manner to produce a thermal image of the surface on the screen. The
luminescent layer is excited to luminescence by repetitive pulses
of excitation radiation, the CCD camera array then analyzing the
transient characteristics of the resulting luminescence.
Such a CCD camera array can be similarly operated as a multiplexer
with a large number of optical fiber luminescent sensor probes by
optically coupling each individual probe through its optical fiber
to one or more CCD photosites that do not receive a luminescent
signal from any other probe. Since a typical CCD camera array
includes well over 100,000 individual photosites, in order to
provide a high level of resolution for two dimensional imaging, its
adaptation as a common detecting and measuring instrument for a
large number of optical fiber probes allows an equal number of such
probes to be used with it. Of course, few temperature measuring
installations would utilize that many individual independent
optical fiber probes but it shows the very high capacity that is
made available as compared with only a handful of such probes
currently being used with a single fiberoptic thermometry
instrument. Alternative to use of a two-dimensional array, a linear
CCD array may be used for the multiplexing application. Linear CCD
arrays are commercially available with from 100 to 1000 individual
photosites. Even if only a small proportion of the total available
capacity of either a two-dimensional or linear CCD array is
utilized, the cost of the system on a per-sensor basis can be
significantly reduced relative to current multi-sensor
instrumentation systems.
Additional objects, features and advantages of the various aspects
of the present invention will become apparent from the following
description of its preferred embodiments, which description should
be taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a remote, real time surface
temperature measuring system that utilizes the present
invention;
FIG. 2 schematically illustrates a CCD camera array of the type
utilized in the system of FIG. 1;
FIGS. 3-7 each illustrate a different specific way of operating the
CCD camera array in the system of FIG. 1;
FIG. 8 is a flow diagram illustrating the processing of the signals
obtained from the CCD camera array in the system of FIG. 1 when
operated in accordance with any of the ways illustrated in FIGS.
3-7;
FIG. 9 shows a modification of the system of FIGS. 1-8 wherein the
luminescent screen is separated from the object surface being
measured and imaged; and
FIG. 10 shows another modification of the system of FIGS. 1-8
wherein a large number of optical fiber ends are bundled together
and imaged onto the CCD camera array in place of the two
dimensional luminescent surface, each optical fiber having a
temperature sensor at its end.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to FIG. 1, a technique and system is generally
described for accurately measuring the temperature profile across
the surface of an object 11. The object 11, shown in FIG. 1 as a
generalized one, is one which has an exposed surface to be
monitored and which has a layer 13 of luminescent material placed
in contact with the surface in order to result in good thermal
transfer from the surface to the layer 13. This contact can be
provided, for example, by painting the luminescent material
directly onto the surface. Alternatively, the layer of luminescent
material can be included as part of an elastic sheet that is placed
onto the object surface. The types of objects represented by the
general object 11 can include aircraft and aerospace models in a
wind tunnel, operating electronic components or electronic
assemblies such as printed circuit boards, portions of the human
body, and the like.
An excitation light source 15 directs light against the layer 13
through an optional filter 17. The resulting luminescence from the
layer 13, is viewed by a CCD camera 19 through an optional filter
21. Since luminescent materials, as is well known, emit radiation
in a wavelength band that is different from the band in which it
absorbs excitation radiation, the filters 17 and 21, if utilized,
are provided for the purpose of isolating radiation of the
excitation source 15 from the camera 19. That is, the filter 17
preferably has a bandpass within the luminescent material's
absorption band so the wavelengths outside of that band do not
reflect off the layer 13 into the camera 19. Similarly, the filter
21, if utilized, is selected to have a bandpass that includes the
luminescence but excludes the excitation wavelengths that pass
through the filter 17. Otherwise, the filter 21 can be a very broad
band filter so that all the luminescent radiation from the layer 13
is received by the CCD camera 19.
An analog output of the CCD camera 19, in a circuit 23, is received
by an electronic processing and control circuit 25. It is in this
block 25 that the raw analog frames of data from the CCD array of
the camera 19 are initially stored, manipulated and analyzed. A
resulting temperature related signal is outputed in a circuit 27 to
a real time monitor or to a data or video recorder 29. A system
controlling computer 31 also receives, through circuits 33, the
resulting temperature related signals. The computer 31 is used to
control the system and may include a monitor 35 and a keyboard 37.
A timing generator 39 is important to the system, coordinating
operation of the excitation light source, over control circuit 41,
the CCD camera 19, over control circuit 43, and the electronic
processing and control circuits 25, over circuit 45. A circuit 47
between the computer 31 and the timing generator 29 allows an
exchange of control and status information between these two
portions of the system. In a more compact version of the system,
the computer 31 may be replaced by a microprocessor chip mounted on
the electronic processing and control circuit 25. In this
situation, the computer monitor 35 would disappear and the keyboard
37 would be replaced by control switches and firmware.
Referring to FIG. 2, operation of a CCD camera array of the type
utilized in the present invention will be very briefly described.
Since such arrays are commercially available in substantial
volumes, practically no physical modification of the commercially
available arrays is required. The most significant adaptation to
available CCD arrays in order make them useful in the applications
being described is in a particular control of their operational
frequency (frame rate) and timing, in a manner described below.
The CCD camera array illustrated in FIG. 2 is not intended to be
complete or comprehensive but rather to provide a background for
the subsequent discussion of its operation. A two-dimensional array
51 of individual photosites generally has them arranged in columns
and rows. Only 19 columns and 14 rows are shown in FIG. 2 for
simplicity. One standard CCD array, however, includes 430 columns
and 488 rows, for a total of 209,840 individual photosites. The
array is constructed as an integrated circuit on a single silicon
substrate less than one inch on a side.
Each CCD photosite builds up an electronic charge in an amount
proportional to the intensity of light striking it and the time
over which the light is incident on it. Alongside each photosite is
a vertical shift register. The electronic charge that is
accumulated in each photosite during a frame of operation of the
array is moved from the array to the electronic processing and
control system 25 by first shifting these analog charge packets in
the vertical shift registers, and thence through a horizontal shift
register 53 and out of the array. Each frame of data consists of a
sequence of signals corresponding to the individual photosite
charge packets, the identity of each individual photosite signal
being determined by its time of arrival with respect to some
reference frame time. Each frame of data is stored in one of
several frame stores 26 and then, at an appropriate time, retrieved
and processed in the manner described below.
As can be seen, a great deal of data is processed as a result of
one frame of operation of the CCD camera array since it will have
over 100,000 individual photosites and thus that many individual
pieces of data. A typical CCD camera frame rate is 60 frames per
second, meaning that all this happens in about 17 milliseconds. The
frame rate of available CCD camera arrays can be adjusted
significantly, however, if necessary to match to the decay time
characteristics of the luminescent material chosen or to otherwise
facilitate the large amount of signal processing that needs to
occur in order for the system to operate in substantially real
time.
Referring to FIG. 3, one of five specific ways of operating the CCD
camera of FIGS. 1 and 2 is described. A first waveform 55
represents the light pulses produced by the source 15 (FIG. 1) in
response to commands from the timing generator 39 over the circuit
41. The light source 15 may be a flash lamp, light emitting diode
(LED), solid state laser, or the like. A resulting luminescent
signal from one small area of the layer 13 is shown by a curve 57.
Each area of the luminescent layer 13 produces an emission pulse
with a unique shape of its time dependence that is dependent upon
its temperature. As can be seen from FIG. 3, the luminescence
signal 57 builds up during the excitation pulse, and then decays in
a substantially exponential manner until a subsequent excitation
pulse, at which time the process begins again.
In the example of FIG. 3, the CCD camera 19 is illustrated to have
successive frames A1, A2, A3, A4, A5, and so on, each having the
same duration. The timing generator 39 is configured to emit an
excitation pulse that ends coincidentally with the end of every
other frame. This occurs at time ta2, at the end of frame A1, and
at time ta5, at the end of the frame A3.
A signal 59 of FIG. 3 shows the output of a single photosite of the
CCD camera array. In this embodiment, any output data during the
frames in which the excitation pulse occurs is sacrificed. Thus,
the photosite output signal 59 is shown to be zero during frames
A1, A3 and A5. However, in an actual operation, the photosite
output signal will likely have some value during each of those
frames but the signal is not retained by the frame stores 26 of the
signal processing circuitry during those frames. Therefore, it is
shown in FIG. 3 to be zero since its value is of no interest.
In order to measure the temperature of the small area of the
luminescent layer 13 (FIG. 1) that is imaged onto the photosite
whose signals are being shown in FIG. 3, data from two successive
periods of decay of the luminescent signal 57 (FIG. 3) are
utilized. As noted by the signal 59, the timing generator 39 (FIG.
1) allows charge to be accumulated by the photosite throughout the
entire duration of the frame A2, resulting in accumulation of a
charge proportional to the area Ia1 under the curve. This is a time
integral of that curve over the duration shown. During the next
frame A4 of the next decaying signal cycle, charge is not
accumulated for the normal frame period but rather this frame is
begun at a delayed time t.sub.D after the beginning of the frame at
time ta5. Thus, the integration period during this cycle is from
time ta6 until time ta7 at the end of the A4 frame.
The resulting integration quantity Ia2 is then compared with the
previous integration value Ia1 by ratioing them, by subtracting one
from the other, by dividing the difference of the two by the sum of
both of them, or by some other appropriate mathematical comparison.
The result of this comparison is then converted to temperature by
use of either a formula or a digital lookup table within the
electronic processing and control circuits 25. Such a formula or
table is empirically determined for the particular luminescent
material being utilized. That is, a number of such measurements are
made over the full range of temperatures likely to be to be
encountered while selected temperatures are also measured by a
thermocouple or other accurate reference sensor in order to acquire
the formula or table which is used in this manner.
FIG. 4 shows a slightly modified version of the system operation of
FIG. 3. A curve 61 shows repetitive excitation pulses which, in
this case, are not periodically occurring as they are in the
example of FIG. 3. Rather, one pulse is caused to appear during a
first portion of one frame B1 while the next pulse occurs at the
end of the frame B3. The next pulse reverts to occurring during the
beginning of the frame B5, and so forth. This results in shifting a
luminescent signal 63 in time with respect to the measurement
frames B2 and B4. Charge is accumulated during these entire frames
but a different portion of the decaying luminescent intensity
signal 63 is integrated during those periods because of this shift
of the relative timings of the excitation pulses. One complete
cycle of operation is shown by the four frames B1 through B4. A
comparison of the integrated values Ib1 and Ib2 allow the
temperature to be measured of the luminescent layer area that is
imaged onto the photosite.
With reference to FIG. 5, a third specific signal processing
alternative is illustrated. In this example, one excitation pulse
is emitted at the beginning at each of the frames C1, C2, C3 and
C4, as shown by a curve 67. The duration of each of the pulses is
made to be a small part of each frame in order to have enough time
remaining to analyze a decaying portion of a luminescent 69 during
each frame. The period of charge accumulation of the photosite
during each frame alternates between them. During a first frame C1,
the signal 69 is acquired for a period beginning at time tc1
immediately upon termination of the excitation pulse. An integrated
signal Ic1 is acquired from that time until a time tc2 which occurs
before the end of the frame C1. In the next frame C2, a different
portion of the decaying luminescent curve is acquired, from a time
tc5 until the end of the frame C2 at the time tc6. It is the
integrated signal Ic2 acquired during the frame C2 that is compared
with the integrated charge value Ic1 acquired during the
immediately preceding frame C1.
Thus, it can be seen from FIG. 5 that a temperature measurement is
made in only two successive frames of the CCD camera array while
four such frames are required for a single measurement in either of
the implementations shown in FIGS. 3 or 4. Of course, this requires
either using a luminescent material with a shorter decay time
characteristic in the implementation of FIG. 5, or a longer video
frame duration, or both, than with the implementations of FIGS. 3
and 4. As is apparent from reviewing the operation waveforms being
described, it is important that the luminescent material be chosen
to have a range of decay times over the range of temperatures to be
encountered that is properly matched with the CCD camera frame
duration in order that enough luminescent signal exist during two
different integration periods.
Referring to FIG. 6, yet another operational technique is
illustrated. As shown by a curve 73 of the excitation pulses, one
pulse is timed to occur every third CCD camera array frame. A
complete operational cycle occurs during three successive frames,
as shown during frames D1, D2 and D3. A resulting luminescent
signal 75 is integrated simultaneously with the occurrence of the
excitation pulse during the frame D1, thus acquiring an integrated
quantity Id1 of an output signal 77 of a single photosite. The
second integration Id2 occurs during an initial portion of the
luminescent decay that occurs during the frame D2. No signals
acquired during the frame D3 in order to allow the luminescent
signal 75 more time to decay to an appropriate beginning intensity.
A comparison of the two integrations Id1 and Id2 thus gives the
temperature. In this technique, the filters 17 and 21 must almost
certainly be used to prevent the CCD camera from being blinded by
the excitation pulses.
A final example of the CCD camera array operation is given in FIG.
7. The excitation radiation pulses shown in a curve 79 are the same
as those in the curve 73 of the FIG. 6 embodiment. Similarly, a
signal 81 of the luminescent emission striking the one photosite
being evaluated is the same as the curve 75 of FIG. 6, assuming
that the luminescent materials and other conditions are all the
same. What is different, as shown by a curve 83, is that the output
of the photosite represented by that curve is utilized during the
final two of a three frame cycle, rather than the first two frames
as shown in FIG. 6. A first part of the decaying signal is
integrated during the entire frame E2, resulting in an integrated
quantity Ie1 that is compared with a quantity Ie2 that is the
integration of the luminescent signal in the next successive frame
E3. A ratio of these two integrated quantities or some other
appropriate comparison thus leads to temperature.
The operational technique illustrated in FIG. 7 is generally
preferred, primarily for the reason that both of the photosite
quantities which are compared in order to make a single temperature
measurement are acquired from a single luminescent decay curve. It
will be noted from the examples of FIGS. 3, 4 and 5 that one of the
two quantities to be compared is obtained from a different
luminescent decay function. That is, each of the two photosite
integration quantities are obtained as a result of a different
excitation pulse. It is preferable to acquire both quantities as a
result of a single excitation pulse, as in the preferred embodiment
of FIG. 7, in order to eliminate any variations that may occur from
excitation pulse to excitation pulse. Any such variations cause
non-temperature variations in the resulting luminescent signal.
This disadvantage of the embodiments of FIGS. 3, 4 and 5 can be
minimized by averaging a number of measurements but the necessity
of doing this requires more time to determine a temperature value.
In applications where the temperature may be varying and such a
variation is desired to be monitored in substantial real time, this
can be a disadvantage. The embodiment of FIG. 6 has the same
advantage as that of FIG. 7 but is not preferred since one of the
photosite integrations occurs during the excitation pulse, thus
being susceptible to the photosite receiving excitation radiation
in cases where the filters 17 and 21 do not adequately suppress the
amount of excitation light reaching the CCD camera.
As previously mentioned, any operable system will have the decay
time characteristics of its luminescent material matched to the
frame rate of the CCD camera that is being used. The decay time of
the luminescent material appropriate for the FIG. 5 embodiment must
be the shortest, or the video frame would rate the longest, of the
five embodiments being described. Conversely, the embodiments of
FIGS. 6 and 7 require the longest decay rate in a luminescent
material, or the shortest frame rate, among the five embodiments
being described.
Although there are a wide variety of luminescent materials with
decay times long enough to be useable in these embodiments, one has
been found that appears to be particularly suitable. This material
is chromium-activated yttrium orthoaluminate (YALO.sub.3 :Cr) in
powder form. This material can be excited in the visible range of
the electromagnetic energy spectrum, with green or blue-violet
light. The luminescent emissions are well separated from the
excitation radiation, being within the deep red and near infrared.
The material has a fluorescent decay time which varies nearly
linearly with temperature from 54 milliseconds at -200.degree. C.
to about 10 milliseconds at +300.degree. C. At +425.degree. C.,
this material has a decay time of about 4 milliseconds. This range
of decay times is suitable for use with a standard video frame rate
of 60 Hz, resulting duration of each frame being about 17
milliseconds. In addition to these favorable characteristics, the
material is thermally stable, melting above 1800.degree. C., and
has other characteristics that are required for a luminescent
material to be useful in any temperature measuring application.
The principal disadvantage of all these schemes is that several
input frames are required in order to output a single frame of
processed data. This may be compensated in part by running the CCD
camera at higher than normal frame rates. A limit will probably be
set, however, by the resolution of the thermal data being
processed. Alternatively, data rates can be increased by ganging
together adjacent photosites, but this obviously decreases spatial
resolution. Another alternative is to obtain the data in real time,
store all the raw data and then, if it is desired to retain both
full spatial and thermal resolution, do the processing later at
whatever speed is required, followed by representing the data
either in real time or slow motion. Clearly, the specific
application will determine the best approach.
The use of data acquired from two fresh frames has been described
in each of the embodiments of FIGS. 3-7 in order to make a single
temperature measurement. That is, the data of each frame is used
only once. The speed of the process can be increased, however, if
some of the frame charge integrals are used in more than one
calculation of temperature. There are two specific ways to do this.
One is a "rolling" processing technique wherein each charge
integral is compared with the immediately preceding charge
integral. Thus, each is used in two successive comparisons and
temperature calculations. The result is twice the number of
temperature measurements in the same amount of time.
The second way of using a frame measurement more than once is to
hold one frame measurement as a reference and then compare each of
successively acquired frame data with that reference for a number
of frames, until finally the reference frame is freshly measured.
For example, with reference to FIG. 3, the charge integral Ia1
could be used as a reference and the shortened period charge
integral Ia2 then measured in each of several successive frames
after that, a temperature calculation being made after each of
these successive frames as a result of comparing the shortened time
charge integral with the reference full frame charge integral Ia1.
Four frame stores 26 provide enough storage capacity to implement
this variation. A disadvantage of this technique is that rapidly
changing temperature is not followed as rapidly as it is with the
methods earlier described, since one quantity being compared as
part of each temperature measurement remains unchanged for several
successive frames.
Another way of speeding up the rate at which temperature
measurements are made is to use two or more CCD cameras that are
operated in staggered sequence. The same image field is focused
onto each of multiple CCD cameras. Even though the operational rate
of each camera is not increased, the rate of temperature
measurements is increased by the overall system.
Each of the operational embodiments of FIGS. 3-7 have assumed that
the video frame rate is fixed over the full range of temperatures
which can be measured. However, in order to take full advantage of
luminescent materials that have a widely varying decay time over a
temperature range of interest, it may be desirable, when such
materials are utilized, to vary the CCD camera array frame rate as
a function of temperature. The reason for doing so is to maximize
the amount of signal that is detected during the frames where
integration occurs. For example, as the decay time becomes very
short with extremely high temperatures, it would be desirable to
shorten the duration of each frame (increasing the frame rate) in
order to better match the signal processing to the decay time being
experienced at the moment. Another approach is to provide manually
or automatically selectable two or more ranges of operation wherein
the frame rates are set to be different when measurements are being
made in each of two or more temperature ranges.
It is desirable to compensate, as part of the data processing, for
any differences in the characteristics of the individual CCD
photosites. Their individual dark currents are determined by
imaging a dark field onto the entire array and storing the
integrated charge measured in one frame. This then becomes data
that is permanently stored in the system. The stored dark current
value for each photosite is subtracted from each measured charge
integral in order to compensate for it. Similarly, the CCD camera
is exposed to a light field of uniform brightness, any variations
in integrated charge readings during one test frame being used as a
multiple of subsequent readings in order to provide additional
compensation for sensitivity variations from photosite to
photosite.
Referring to FIG. 8, a brief flow chart illustrates the nature of
the calculations done by the processing and control system 25 (FIG.
1), according to one method, for any of these specific embodiments
of FIGS. 3-7. The processing begins, in steps 85 and 87, by
acquiring the frames of integrated charges I1 and I2 in accordance
with any of the five embodiments of FIGS. 3-7. These two sets of
values are then digitized and, in a step 89, the values are
compared, preferably by taking a ratio, but other comparison
techniques as mentioned previously can be utilized. This ratio is
then used in a step 91 to calculate or look up the temperature
using an empirically determined formulation or look-up table, as
previously discussed. The resulting temperature of each small area
of the object surface is used to form a display or is recorded.
Several alternative ways have been described for determining
temperature of a single point of the luminescent material 13 that
is imaged onto the single CCD camera photosite. The same process
occurs for each of the other photosites, depending on the size of
the CCD camera array that is utilized. The temperature calculation
for each such image point can then be utilized in a number of
different ways. The raw data, in computer form, can be analyzed,
the temperature values then converted to values of color and
displayed on a color monitor, or recorded by a color printer, to
show surface temperature variations as variations in pseudo-color.
These temperature values can also be used for industrial control,
some action occurring in response to the occurrence of a certain
temperature pattern. The applications are numerous when precise
temperature data are made available. By using commercially
available CCD camera arrays, the data is obtained in a form that
can easily be utilized by either standard computer or video imaging
equipment.
Referring to FIG. 9, an alternative implementation is described. An
object surface 101 is desired to be monitored and its temperature
measured. Rather than coating the luminescent material directly on
the object surface, however, as was done in the embodiment of FIG.
1, a thin layer 103 of luminescent material is included as part of
a screen 102 that is physically removed from the object surface
101. The screen 102 includes an infrared absorbing material layer
104, such as a blackened layer of an appropriate material. The
infrared emissions from the object surface 101 are then imaged by
an appropriate optical system 105 onto the layer 104. The layers
103 and 104 of the screen 102 are held in intimate contact with
each other in order to provide for good thermal transfer from the
infrared image absorbing layer 10 to the luminescent layer 103. The
luminescent layer is thus heated in accordance with the infrared
image from the object surface. The thermal mass of the screen 102
is made to be as low as possible in order to allow it to rapidly
respond to changes in the infrared image on the layer 104.
The luminescent layer 103 is caused to luminesce by an excitation
source 107. The visible or near visible luminescent emissions from
the layer 103 are imaged by an appropriate optical system 109 onto
a CCD camera system 111. A pair of optical filters may optionally
be employed, similar to the system of FIG. 1. The CCD camera system
111 is of the type previously described with respect to FIGS. 1-8.
Operation of the system of FIG. 9 is also the same except that the
screen 102 serves to convert the heat image of the surface 101,
instead of placing the luminescent material in direct contact with
the surface of interest.
An advantage of the system of FIG. 9 is that a true infrared camera
results. All of the elements shown in FIG. 9 (except, of course,
the object's surface 101) are packaged together as a single
instrument. This instrument can be of very small size and low cost
when compared to existing infrared cameras. The system can operate
at room temperature and no mechanical scanning is required.
However, calibration of the system does still require a method of
relating temperature variations in the image plane (screen 102) to
the temperature variations of the surface 101, a problem common to
all infrared cameras (thermographs).
Referring to FIG. 10, a different use of a similar type of CCD
camera system 113 is explained. Rather than luminescence from a two
dimensional surface being imaged onto the CCD camera array, a large
number of optical fibers are formed into a bundle 115 so that their
ends terminate in a common plane which is then imaged by an
appropriate optical system 117 into the CCD camera 113. Each of the
optical fibers within the bundle 115 can carry a separate,
independent luminescent optical signal whose decay time is to be
measured. In a preferred embodiment, each of the optical fibers
terminates in a temperature sensing probe, such as a fiber 118
having a temperature sensing probe 119 formed at its end. The
temperature sensing probe 119 contains luminescent material, as
currently commercially utilized and discussed in the literature. In
this use, a number of independent sensors can be utilized that are
equal to the number of photosites within the CCD array of the
camera system 113. A tight bundle of the optical fiber ends is then
imaged onto that array by the optics 117 in a manner that each
optical fiber end is imaged onto a unique one or more of the CCD
photosites. As an alternative to the CCD camera with a
two-dimensional array of photosites, a linear array may be used for
this application. In either case, the system is as much a
multiplexer as an imager, the image simply being used to identify
each sensor by its location in the image.
In order excite all of the luminescent sensors at the end of the
large number of optical fibers, an excitation source 119
periodically generates pulses which are focused by an optical
system 121 onto the exposed ends of the fibers within the bundle
115. A dichroic beam splitter 123 is utilized to reflect the
excitation light onto the fibers while at the same time allowing
the different wavelength band of the luminescence to pass through
it and onto the CCD camera system. Optical filters 125 and 127,
having nonoverlapping wavelength bandpass characteristics, are
optionally employed in order to eliminate, to the extent possible,
radiation of the excitation source 119 from reaching the CCD camera
113. When employed, the filter 125 passes a wavelength band of
electromagnetic radiation to excite the luminescent material of the
sensors, excluding all else, and the filter 127 passes another band
of wavelengths corresponding to that of the luminescence from the
sensors, excluding all else.
As mentioned previously, the advantage of this approach is that a
very large number of optical fiber luminescent probes can be
utilized with a single CCD camera array based instrument. The
complexity and cost of a multi-sensor temperature measuring system
is thus reduced significantly.
Although the invention has been described with respect to its
preferred embodiments, it will be understood that the invention is
to be protected within the full scope of the appended claims. For
example, even though the preferred embodiments deal with only
temperature measurement, it will be understood that the measuring
system of this invention is equally useful for the measurement of
other parameters. The only requirement is that the parameter being
measured somehow affects the rate of luminescent decay in a
measurable way. The system of the present invention then provides
an improved way of measuring that rate of decay. The novel use of
the CCD camera array according to the present invention has
application anywhere there is a decaying optical signal whose
properties are desired to be utilized for either visualization or
measurement. The signal could, for example, be from a fluorescent
gas layer and the data obtained could be used to acquire flow
information rather than temperature alone.
* * * * *